After reading this definition it seems strange to suggest that rancid is something that chefs should be considering as a taste to use in the kitchen, but that’s exactly what I’m about to do.

First consider rancid – do you wince a little? Twist your lips in a lemon-sucked-style? Spin your head to the side in disgust? If you’ve grown up with a standard Western diet then it’s a fair to say that the idea of rancid food is probably something that repels you more than it attracts. But if you’ve grown up in the Faroe Islands eating rast, a lamb that’s hung without preservatives for up to two years; in Morocco consuming salted smen, a fermented butter; or farming Yaks in Tibet, living off the constantly evolving butter of your herd, your idea of rancidity will most likely be very different.

“Rancidity exists in many different cultures," says Roberto Flore, head of culinary research at TheNordic Food Lab(NFL) and a man who has spent the past few months in a kitchen surrounded by varying degrees of rancid butter. “The cultural value was the first engine for starting this research. Trying to understand more why people like some products and why others really don’t like them. If you are able to explain this is a natural process, how it happens, then you start to create the right steps to give the tools to the people to understand that specific flavours also need to be connected with important cultural messages and specific areas of the world. At this point you can connect more cultures and create a deeper understanding of what is actually edible.”

The rancid research carried out at NFL was conducted by Johnny Drain, a research intern with a PhD in Materials Science from the University of Oxford. It consists of four concise papers analysing the scientific, cultural and culinary impact of rancidity, with a particular focus on rancidity in butter.

Samples from the Nordic Food Lab.

In the simplest possible terms, Drain describes rancidity as the decomposition of triglycerides by hydrolysis and/or oxidation. This process creates free fatty acids which can break down further into a wide array of flavourful compounds. One of the triglycerides found in butter is that of butyric acid and when this breaks down, largely thanks to oxidation, it is liberated and produces a unique smell. A smell that helps contribute to the flavour and odour of foods like Parmesan, goat’s cheese and kombucha. However, Drain points out that in high amounts, butyric acid can easily go the other way – explaining that it’s used to make stink bombs and is also the primary smell in human vomit. It’s a very thin line between tasty hit and some pretty terrible shit.

“The work we were doing here in the lab was to search on purpose for rancidity," says Flore, “we worked in creating many different situations where rancidity is happening, as something that we are really searching for.” This was done through the creation of lots of different trial butters. “I still have three of them,” he says, “they’re still evolving. There is one that smells like an old cow’s cheese and one that smells a lot like animal – then the challenge is trying to find the right dish for each flavour.”

Chefs in high end gastronomy have already started to experiment with rancidty as a taste, but this is still small scale. Michel Bras has published a few recipes that call for the use of oils that are made rancid by added pork lard. Andoni Luiz Aduriz at Mugaritz is currently surprising guests with a dessert that consists of a sweet Michelin-man marshmallow sitting in an acidic, tongue cutting sauce of oxidised wine soup. Flore himself recounts a dish from Portugal’s Leonardo Pereira who serves a simple raw oyster with sea fennel and Pata Negra fat on top – it’s the fat, says Flore, that gives the dish a natural rancidity that provides an interesting flavour. “At this point rancidity works perfectly – it’s giving this roundness to the dish that otherwise was probably too sharp and too easy in terms of association – he is creating a complexity with an ingredient that is very common.”

Flore summarises the idea behind the research well: “We want to understand in what situations rancidity is happening and at that point, once we understand what are the criteria to control rancidity, we can observe some rules.” The idea is that, if the process of rancidity can be somehow controlled, then these new flavours could help provide a whole new set of notes for chefs to use when composing dishes.

As Drain asks in the summary of his second paper: “Could we prioritise one rancidity pathway over others to produce a desired flavour profile? Could we stabilise a mildly rancid butter in a given state using an appropriate combination of antioxidants or storage conditions? … Could we reframe rancidity in butter as a positive quality?”

In the lab they’re now working on incorporating the findings of Drain’s research into a number of different approaches. One is to age meat underneath rancid butter in the hope of transferring some of the flavour into the meat. They’re also working with rancid milks and creams, as well as playing with specific mushrooms that can be used to break down fats and take on rancid elements.

Flore says he wants to see more restaurants using these techniques, and that he would love to see the day where a chef tastes a dish and asks for a little more rancidity. “I think in the gastronomic world where customers are already into a more open and complex way of understanding food, you could introduce these rancid elements to dishes and maybe start to serve these products with something that’s very comfortable, like the oyster dish.”

In 2012, Washington Univeristypublished researchthat argued the sixth taste, after umami, is that of fat. In a 2015 study by Purdue University, this idea was confirmed and the taste was named 'oleogustus'. Flore says to consider rancidity as a taste in its own right is not really fair, "it's more of a nuance... a way of exploring the potentials of the sixth taste."

Thinking of fats in this way could add new layers and depth to dishes, though it's a challenging concept to accept, it will be great to see if any chefs incorporate these ideas into their cooking, after all, isn’t great dining also about discovery?